1. Field of the Invention
This invention relates to an automatic fine tuning (AFT) circuit suitable for controlling the local oscillator frequency in the tuner of a color television receiver, and is particularly directed to such an AFT circuit having an expanded capture range for finely tuning to broadcast-frequency video signal.
2. Description of the Prior Art
Because fine tuning of a color television receiver is important in achieving proper color balance in a color television picture, most color television receivers incorporate an automatic fine tuning (AFT) section so that the receiver will tune itself. The AFT section tunes the local oscillator of the tuner to a frequency corresponding to an exact tuned position with respect to the band pass characteristic of the receiver's intermediate-frequency (IF) stage. With such an AFT circuit, the picture and sound remain correctly tuned despite any signal drift or tuning misadjustment.
Generally, AFT circuits include an error sensor and a control device. The error sensor can be a discriminator tuned to the video IF carrier frequency (typically 45.75 MHz) and providing a DC error voltage corresponding to the deviation of the video IF signal about or below that frequency, within a capture range of above .+-.1.2 MHz. The control device can typically be a voltage-controlled oscillator associated with the local oscillator to shift the local oscillator frequency in accordance with the DC error voltage.
When a video signal is generated from a video tape recorder (VTR), video disc player, video game, home computer, or other device and is to be monitored on a color television receiver, the video signal is typically modulated onto a broadcast-frequency carrier signal, so that the generated video signal can be applied to the tuner of the receiver. This is usually done on an RF modulator incorporated in the device.
For example, in the New York City viewing area, broadcast channels 2, 4, 5, 7, 9, 11, and 13 are assigned to different VHF television broadcast stations, with the intervening channels, i.e., channels 3, 6, 8, 10, and 12 not being assigned so as to minimize the chance of adjacent-channel interference. Accordingly, the generated video signal can be modulated onto the broadcast frequency corresponding to one of these unassigned channels, e.g., channel 3.
On receiving the video signal modulated onto the frequency of channel 3, the signals of adjacent channel 2 and 4 are attenuated and the modulated channel 3 signal is received and processed in the receiver.
However, if this unassigned channel 3 is selected by the receiver's channel selector, but the RF modulator of the VTR or other device is not operating, the AFT circuit will hunt for the center frequency of the sound signal for the next-lower adjacent channel, i.e., channel 2. If that frequency is within the AFT capture range, the sound carrier frequency of channel 2 will be pulled in by the AFT circuit.
Therefore, once this has happened, if the RF modulator commences operation, the broadcast-frequency video signal produced thereby (i.e., channel 3) will be above the capture range of the AFT circuit which is tuned to the channel 2 sound carrier. Thus, it can occur that the receiver will appear unable to receive the broadcast-frequency video signal from the VTR or other device.
When the foregoing occurs, the user might surmise that either the receiver or the VTR is out of order, although the problem can usually be cleared by re-selecting the desired channel (in this case, channel 3).
The above-described AFT misoperation can also occur where the television receiver is connected to a cable system in which every channel is used and, for some reason, the signal on one channel is interrupted.
In order to prevent this misoperation from occurring, it is possible to reduce the AFT capture range. However, an AFT circuit with a narrow capture range will not operate reliably if there is an unusual amount of tuner drift, for example, due to temperature changes, and will not pull in a video cable signal where the carrier frequency is offset somewhat from the standard television broadcast frequencies.